In order to meet higher capacity demands and higher user experiences, heterogeneous networks (hetnets) are considered as an important complement to densification of macro networks. Heterogeneous networks can be characterized as deployments with a mixture of macro cells and small cells with overlapping coverage areas. One example of such deployments is where small, so-called pico cells are deployed within the coverage area of larger macro cells to offload macro traffic and to provide higher bitrates by reducing the distance between users and the serving base station. A pico base station is an example of a low power node (LPN) transmitting with low output power, as compared to a high power node, and thus typically covers a much smaller geographical area than a high power node, such as a macro base station.
Small cells and macro cells can be deployed on same frequency or on separate frequencies. In scenarios with co-channel deployments, i.e. small cells operate on the same frequencies as the macro cells, there will be a link imbalance in best cell association for downlink (DL) and uplink (UL) transmissions. Typically, a UE will associate to the strongest cell, i.e. to the cell with the highest received DL power, which means that the DL coverage area is dominated by the macro cell. In the UL, however, the “best” cell is usually determined by lowest path loss. Hence, there will be a mismatch between the “best” cell for UL and DL transmissions as illustrated in FIG. 1. To some extent this can be alleviated by adding a bias term when cell association is done. By biasing the DL measurements, the network can associate a UE to a LPN even if the measured power is higher from the macro cell (HPN). By this, the coverage area of the LPN is increased, hence the term “range expansion”. Cellular networks like 3GPP LTE have been designed for operations with a certain amount of range expansion (handover bias) which may, however, not be sufficient to achieve efficient operations of heterogeneous deployments with large power differences between LPNs and high power nodes.
One challenge associated with range expansion is the coverage of DL physical control channels transmitted from the LPN in the small cells, as reliable reception of the physical control channels is essential for data communications. In LTE, the Physical Downlink Control Channel (PDCCH) or/and the enhanced PDCCH (ePDCCH) carry the DL control information needed by a UE to receive and transmit data. With large range expansion, the inter-cell interference (ICI) from the macro cells could be excessively severe and prevent reliable detection of these control channels. A solution to this could be to introduce almost blank subframes (ABS) or reduced power subframes (RPSF), where UE specific transmissions in certain subframes from the macro node are either muted or transmitted with lower power. By this, the probability of detecting physical control channels will increase.
The principle of ABS/RPSF is illustrated in FIG. 1b. In this case an interfering macro cell mute or reduce transmission power on data to macro users in certain subframes, in order to create protected radio resources for the pico cell. The macro base station indicates via the LTE inter-node interface X2 to the neighbor pico base station the subframes it intends to mute or reduce transmit power. The pico base station can then take this information into account when scheduling users operating within the cell range expansion zone; such that these users are prioritized to be scheduled in protected subframes, i.e. low interference subframes. Pico users operating near the pico base station may in principle be scheduled in all subframes. One may notice that ABS/RPSF assumes that pico cells are time synchronized to the macro cell, as a request for creating protected subframes.
However, a consequence of applying ICIC schemes like ABS/RPSF is the reduced capacity of the cooperating macro cells. Some investigations show that the net gain of introducing ABS/RPSF can be very small, or in some scenarios it actually reduces the overall system capacity.